The present invention relates to a method of manufacturing a chip positive temperature coefficient (hereinafter referred to as xe2x80x9cPTCxe2x80x9d) thermistor using electrically conductive polymer having a PTC characteristic.
A PTC thermister composed of electrically conductive polymer is used as an overcurrent protective element in a variety of electronic devices. An operating principle is such that the electrically conductive polymer having a PTC characteristic heats up by itself when an excessive current flows in an electric circuit, changing a resistance of its own into a high value due to a thermal expansion of the electrically conductive polymer, thereby attenuating the current into a safe minute region.
A PTC thermistor of the prior art will be described hereinafter.
Japanese Patent Laid-open Publication, No. H09-503097 discloses an example of a chip PTC thermistor of the prior art. It is a chip PTC thermister comprising a PTC element having a through-hole penetrating between a first surface and a second surface, and first and second conductive members in a layer form, positioned inside of the through-hole, and connected physically as well as electrically to the first surface and the second surface of the PTC element.
FIG. 15(a) is a sectional view illustrating a chip PTC thermistor of the prior art, and FIG. 15(b) is a plan view of the same. In FIG. 15, a reference numeral 81 represents an electrically conductive polymer having a PCT characteristic, reference numerals 82a, 82b, 82c, and 82d represent electrodes composed of metallic foil, and reference numerals 83a and 83b represent through-holes. Reference numerals 84a and 84b are conductive members formed by plating on insides of the through-holes and over the electrodes 82a, 82b, 82c, and 82d. 
A method of manufacturing the abovedescribed chip PTC thermistor of the prior art will be described with reference to FIGS. 16(a) through 16(d) and FIGS. 17(a) through 17(c) which are procedural drawings showing a method of manufacturing the chip PTC thermistor of the prior art.
First, polyethylene and carbon as electrically conductive particles are blended, and a sheet 91 shown in FIG. 16(a) is formed. Next, the sheet 91 is sandwiched with two metallic foils 92, as shown in FIGS. 16(b) and 16(c), and an integrated sheet 93 is formed by thermal-compression molding.
Next, through-holes 94 are perforated in a regularly arranged pattern on the integrated sheet 93, as shown in FIG. 16(d), after it is irradiated with an electron beam. A plated film 95 is then formed on the insides of the through-holes 94 and on the metallic foils 92, as shown in FIG. 17(a).
Etched grooves 96 are formed next in the metallic foils 92, as shown in FIG. 17(b).
The laminated product is now cut into individual pieces along cutting lines 97 of a longitudinal direction and cutting lines 98 of a lateral direction as shown in FIG. 17(b), to complete manufacturing of a chip PTC thermistor 99 of the prior art as shown in FIG. 17(c).
However, there has been a problem as described hereinafter with the conventional method of manufacturing the chip PTC thermistor, when a protective coating is formed on the plated film 95 for the purpose of preventing a short circuit and the like.
That is, formation of the protective coating needs to be carried out only after a pattern is formed by etching the metallic foil 92. Therefore, the protective coating is formed by screen-printing and thermally curing an epoxy base resin, after etched grooves are formed in the metallic foil 92. The problem occurs in this process that a crack may develop in the plated film 95 formed in the through holes 94 due to a mechanical stress generated by thermal expansion because of the heat applied when thermally curing the sheet 91.
It is conceivable to use a method wherein the etched grooves 96 are formed in the metallic foil, the protective coating is formed next, and the plated film 95 is formed thereafter, in order to prevent the crack from developing in the plated film 95. However, a problem has yet remained unresolved that the plated film 95 can not be formed uniformly on inner surfaces of the through-holes 94 in this method. It is presumed that this is because a surface of the sheet 91 loses an electric conductivity, as a result of the heat during the thermal setting of the protective coating, which causes polyethylene element in the sheet 91 to migrate toward the surface of the sheet 91 exposed on the inner surfaces of the through-holes 94.
An object of the present invention is to solve the foregoing problem of the prior art method, and to provide a method of manufacturing a chip PTC thermistor having superior reliability of connection, as it does not cause a crack in the electrode connecting between upper and lower electrodes when the protective coating is formed on the metallic foil, and it is capable of uniformly forming a film by electrolytic plating even on a portion of the electrically conductive polymer on an inner surface of the opening when the electrode is formed.
A method of the present invention for manufacturing a chip PTC thermistor comprises includes the steps of;
forming a sheet by sandwiching an upper surface and a lower surface of an electrically conductive polymer having a PTC characteristic with metallic foils, on which a pattern is formed in advance, and integrating them by thermal-compression molding;
providing an opening in the integrated sheet;
forming a protective coating, also serving as plating resist, on an upper and lower surfaces of the sheet in which the opening is provided;
forming an electrode by electrolytic plating on the sheet on which the protective coating serving also as plating resist is formed; and
cutting the sheet, on which the electrode is formed, into individual pieces.
In addition, a material that is capable of being formed at a temperature below a melting point of the electrically conductive polymer is used for a material of the protective coating, also serving as plating resist, in the step of forming the protective coating also serving as plating resist. Furthermore, a processing temperature is maintained in such a manner as not to exceed the melting point of the electrically conductive polymer in each step of the preparatory processes from the step of providing the opening in the integrated sheet, up to the step of forming the electrode by electrolytic plating on the sheet, on which the protective coating serving also as plating resist, is formed. The manufacturing method of the present invention provides the chip PTC thermistor having a superior reliability of connection, since it does not cause a crack in the electrode formed by electrolytic plating, and is capable of uniformly forming a film of the electrolytic plating even on the portion of the electrically conductive polymer on the inside surface of the opening when the electrode is formed. In addition, the present invention can eliminate waste liquid that is otherwise produced if wet patterning is used for the metallic foil in the process of manufacturing the chip PTC thermistor, since the present method uses the metallic foil patterned in advance by die-cutting to manufacture the integrated sheet.